As far as I know you don't need permission but you must notify the department and provide proof of your earnings - can be a payslip, contract or a letter from your employer. There is a Benefit of Work estimator tool available on My Welfare to allow you to work out how much your allowance will reduce. Earnings up to €120 are not taken into account in the means assessment. If the job does not work out your disability allowance will be re-instated fairly quickly. You can also keep your medical card for 3 years.